1. Field of the Invention
The invention pertains to equipment for glow-discharge treatment in accordance with the characterizing clause of patent claim 1.
2. Prior Art
The technology of plasma carburization has been known for many years and was described, for example, in DE Patent 668,639. To ensure a stable glow discharge for the treatment in the furnace, the two electrodes of the glow potential (anode and cathode) are designed to be electrically insulated from the others parts of the equipment. However, this makes it necessary for the charging plate to be electrically separated from the other parts, at least during the glow treatment. In the equipment of the above-mentioned type that is described in U.S. Pat. No. 4,179,618, the charging plate rests on two electrically insulating supports, which are mounted on the base of the chamber that can be evacuated. Between the two supports, there is a support insulator, which is attached to an electrical bushing insulator and can be connected with a power source for the glow potential.
In the equipment described in German Patent 31 01 351, the charging plate is also supported in the furnace in an electrically insulated way; it is part of a carriage that can be moved in and out of the furnace chamber on electrically insulated rails. In its end position it comes into contact with the supply line for the electrical connection of the charging plate.
GB-A-2 091 398 describes equipment for glow-discharge treatment, in which the workpiece, before being treated, rests on a support that is used to move the workpiece in and out of the furnace. To perform the treatment, the workpiece is raised from the support by a lifting device, which simultaneously connects the workpiece with an electric supply line.
A disadvantage of these previously known types of equipment for glow-discharge treatment is that the charging plate is always supported in an electrically insulated way as long as it is located in the open inner space. This means that the electrical insulators are exposed and can thus become dirty, sooty etc. during the entire treatment operation, i.e., not merely during the actual glow-discharge treatment, so that a sparkover on one or the other insulator cannot be avoided after a certain amount of time.
Insulators for glow discharge treatment are also known (see, e.g., U.S. Pat. No. 4,221,972), in which fouling of the external, exposed surface of the insulator is supposed to be prevented by a large number of peripheral grooves and by gap effect. However, it has been found that the surfaces of these insulators also eventually become covered with a layer that possesses a certain amount of electrical conductivity. Compared to plasma nitriding, the problems encountered in plasma carburizing are more serious because the treatment temperature in the furnace is in a range at which ceramic materials are usually already beginning to conduct. Therefore, the insulators must be located outside the furnace or they must be cooled; however, this leads to additional problems.
The previously known types of equipment have still another important disadvantage, which arises during the heating of the charging plate and when more than one support insulator is present. The charging plate and the workpieces it supports are introduced at room temperature into the chamber that can be evacuated or its furnace. In the furnace it is heated to a temperature of about 1000.degree. C. which causes strong expansion. This in turn produces a transverse load on the supports that support the charging plate. In typical equipment of the type described at the beginning, the charging plate, including the workpieces it holds, weighs perhaps one metric ton. This means that each support must bear a load of several hundred kg, depending on its design. The transverse load described above must also be considered when the charging plate undergoes thermal expansion. This load often leads to the buckling, breaking off or lateral cracking of the supports.
To summarize, the disadvantages of the previously known equipment of this type are, first, that the supports must remain electrically insulating for as long a time as possible, and second, that the supports must be designed to withstand mechanical loads, especially transverse loads during the thermal expansion and subsequent contraction of the charging plate. Standard ceramic materials are unable to withstand these loads. Therefore, in equipment of the types described above, the insulating supports must be replaced relatively frequently because they have been mechanically destroyed or are no longer electrically insulating. This means a lengthy operational shutdown in either case.
Therefore, the goal of the invention was to develop equipment of the type described at the beginning in such a way that the supports would have significantly longer service life, preferably a practically unlimited service life, and that the insulators would be subject to as little mechanical stress as possible and would be well protected from soiling.
This goal is accomplished by connecting the supply line to a head section of at least one support insulator, which terminates below the charging plate; furthermore, the one or more supports or the one or more support insulators can be moved up and down, so that the charging plate rests only on the one or more supports or only on the head sections of the one or more support insulators.